1. Technical Field
The present disclosure relates to a giant magnetoresistance material and, in particular, to a giant magneto-resistance composite material containing carbon nanotubes.
2. Discussion of the Related Art
Since the giant magnetoresistance (GMR) in metallic magnetic multilayers (M. N. Baibich et al., Phys. Rev. Lett. 61, 2472 (1988)) and later in immiscible magnetic granular alloys were discovered, it has aroused great interests in both theoretical and applied magnetism. GMR effect refers to a large resistance change of the GMR material when a magnetic field is applied to the GMR material at a certain temperature. Because it has tremendous practical value in the field of computer hard disk read heads, magnetic sensors and magnetic recording, GMR materials research and application development has become one of the hot spots in the current condensed matter physics and materials science.
The magnetoresistance (MR) of GMR material is given by the formula of:
            M      ⁢                          ⁢      R        =                  (                              ρ            ⁡                          (              H              )                                -                      ρ            ⁡                          (              0              )                                      )                    ρ        ⁡                  (          0          )                      ,wherein ρ(0) is a resistance under the magnetic field intensity equals to zero, ρ(H) is the resistivity under nonzero magnetic field, respectively.
Traditional GMR material can be divided into two types. One is metallic magnetic multilayers. The other is the granular composite films with ferromagnetic grains embedded in insulating matrix.
A multilayer structure of metallic magnetic multilayers can be formed by the appropriate ferromagnet (such as Fe, Co, Ni) and non-ferromagnet (eg, Cu, Cr, Ar) overlapped with each other. Since the magnetocrystalline anisotropy, when applying a magnetic field to the metallic magnetic multilayers, its magnetization curve along the easy axis and hard axis is inconsistent. The MR of this kind of the GMR material reaches 10% at low temperatures (for example 4.2 Kelvin, equals minus 269 degrees Celsius). However, the MR need for the practical application of GMR materials is more than 5% at normal temperature. This kind of GMR material only at low temperatures has the GMR effect, which narrows the scope of their application. And the metallic magnetic multilayers are made of metal, which are difficult to cut and also limit their application.
The GMR effect has been widely explained by spin-dependent scattering theory. Subsequently, GMR was also observed in granular composite films with ferromagnetic grains embedded in insulating matrix. Different from the metallic magnetic multilayers mentioned above, this GMR was caused by spin-dependent tunneling, and so called tunneling magnetoresistance (TMR). The granular composite films are also made of metal, which is difficult to cut, also limits its application.
Carbon nanotubes (CNTs) are novel carbonaceous materials and received a great deal of interest since the early 1990s. Carbon nanotubes have interesting and potentially useful heat conducting, electrical conducting, and mechanical properties. Nowadays, a GMR material with carbon nanotubes is provided via sputtering Co—Zn—P, Co—Fe—P, Ni—Co—P or Ni—Zn—P on the surface of the carbon nanotubes. But this kind of GMR material is only available in a powdered form, which limits its application.